The US president (as commander in chief) has the power to order for military action against a nation, like for example the war on Iraq. The war was not officially declared (because then Bush would have had to get a permit from the congress), but was the cause of the deaths of 500,000 of our children and over one and a half a million muslims totally. So will Donald Trump be able to accelerate the efforts, for a while, to murder muslims in the middle east? Most likely.
But let's ask another question: What effect will Donald Trump have on US and EU? Trump has been flirting a lot with Russia, the to-be president of US gave Putin an A on leadership, I kid you not. He also said that some NATO members should Not get military support, and thereby undermining NATO, because the whole point of NATO is that if one is attacked, all will defend. This is very dangerous for the west as Russia is to it an open enemy. For the US president to hold such views would be like a gift from heaven for Russia. Although I doubt Trump will be able to, in any direct manner support Russia, but he will most likely do something.